Deuterium (hydrogen-2, symbol ²H or D, also known as heavy hydrogen) is one of two stable isotopes of hydrogen (the other is protium, or hydrogen-1). The deuterium nucleus, called a deuteron, contains one proton and one neutron, whereas the far more common protium has no neutrons in the nucleus. Deuterium has a natural abundance in Earth's oceans of about one atom of deuterium among every 6,420 atoms of hydrogen (see heavy water). Thus deuterium accounts for about 0.0156% by number (0.0312% by mass) of all hydrogen in the oceans: 4.85×10¹³ tonnes of deuterium – mainly in form of HOD (or ¹HO²H or ¹H²HO) and only rarely in form of D₂O (or ²H₂O) – in 1.4×10¹⁸ tonnes of water. The abundance of deuterium changes slightly from one kind of natural water to another (see Vienna Standard Mean Ocean Water).